There have recently evolved voltage source converters that are based on cascaded voltage source converter cells provided in phase legs. These provide discrete voltage levels that can be combined for conversion between AC and DC. Each cell is here made up of an energy storage element, typically a capacitor, being connected in series with two switching elements for forming a half-bridge converter cell. These cells typically have two connection terminals, where a first is provided in the junction between the two switching elements and a second in the junction between one of the switching elements and the energy storage element. The placing of the second terminal in the half-bridge cell defines the cell type, where it may be placed at the junction between one of the switching elements and the energy storage element. The placing of the second terminal at a first such junction therefore defines a first type of half-bridge cell, while the placing of the second connection terminal at a second junction defines a second type of cell.
These types of cells are generally described in relation to a voltage source converter in DE 10103031. This document also discloses how half-bridge cells of the same type can be provided in pairs in a phase leg, where one cell in a pair has one orientation and the other cell in the pair has the opposite orientation. This means that a first terminal of a first cell in a pair is connected to the second terminal of a second cell in the pair.
Other documents describing the same principle are “A new modular voltage source inverter topology”, by A Lesnicar and R Marquardt, UNIV. BUNDESW. NEUBIBERG 2003, XP002447365, 10th European Conference on Power Electronics and Applications, 2-4 Sep., 2003, Toulouse, France as well as “Modulares Stromrichterkonzept für Netzkupplungsanwendung bei hohen Spannungen” by Rainer Marquardt, Anton Lesnicar and Jürgen Hildinger, UNIV. BUNDESW. NEUBIBERG 2002, XP002447360, Bad-Nauenheim, Germany.
Further documents describing this area are WO 2007/033852, which is directed towards controlling valve leg currents in a power converter as well as WO 2008/067785 which describes the control in a voltage source converter for regulating circulating currents. In both these latter documents cells of the same type are used that have the same orientation in relation to each other.
The above described modular converter structure provides adequate conversion ability. However, it would be of interest to modify the structure so that enhanced functionality is possible and simplified at a low additional cost.